Due to improved storage capacity, more frequent rechargeability and higher energy densities, there is an increasingly broad range of applications for batteries. Batteries with lower energy storage capacity are used for example for small portable electronic devices, such as mobile telephones, laptops, camcorders and the like, whereas batteries with high capacity are used as an energy source for the drive of motors of hybrid or electric vehicles, etc. or as stationary batteries.
Batteries can be formed for example by the series connection of battery modules, wherein parallel connections of the battery modules are also used in part and the battery modules may in turn consist of battery cells connected in series and/or parallel.
For the drive of motors of hybrid or electric vehicles, multi-phase battery systems in particular are suitable and are also referred to as battery direct inverters (BDIs). Multi-phase battery systems comprise at least two battery module banks connected in parallel, each bank comprising one or more battery modules connected in series. Here, the battery modules have a coupling unit and can be connected and disconnected individually with the aid of said coupling unit, sometimes also with the possibility of choosing between a positive and negative connection. By connecting and disconnecting the modules accordingly, the multi-phase battery system can therefore be used to generate a number of alternating voltages phase-shifted in relation to one another. With suitable design, three alternating voltage profiles phase-shifted in relation to one another can be generated for example, which each approximate a sine curve and can be used directly to drive electric or hybrid motors.
Battery management systems are used for battery management, for example for the basic control of modules, in order to increase the safety of batteries, in order to increase efficiency and in order to extend the service life of battery modules and battery systems composed of battery modules. One purpose of battery management systems is to determine the current state of the battery modules. The information which is key for this purpose includes the impedance or the internal ohmic resistance of the battery module or of the battery cells contained in the battery module, wherein the impedance is dependent on the state of charge, the temperature and the aging of the battery cells.
According to the prior art, associated measurements are taken outside the normal operation of the battery. For example, it is proposed in ISO 12405 to subject batteries that are to be tested to pulsed charging and discharging currents for periods of time lasting for a matter of seconds and to measure the voltage across the battery before and after the current pulse. The impedance is then given as the ratio of the difference of the measured voltages to the strength of the current pulse.
Since the current pulses are applied for one or more seconds, the two voltage measurements are also distanced from one another by this period.